1. Field of the Invention
This invention relates to a control device for a washing machine, detecting the turbidity of a wash liquid contained in a wash tub and automatically determining a period of time of a wash step on the basis of the detected turbidity.
2. Description of the Prior Art
A microcomputer-based control device has recently been incorporated in washing machines. The microcomputer determines a rate of change of the turbidity on the basis of a turbidity detection signal generated by a turbidity sensor provided for detecting the turbidity of a wash liquid in a wash step. Based on the result of determination about the rate of change of the turbidity, the microcomputer automatically sets a period of time of the wash step. The prior art has provided two methods regarding the above-described control: in one method, the wash step time period is determined mainly in accordance with the concentration of a used detergent, while it is determined mainly in accordance with the degree of soil of the clothes in the other method. The detected turbidity is used as information of concentration of the detergent in the former method while it is used as information of degree of soil of the clothes in the latter.
Generally, the detergent concentration at an initial stage of the wash step depends upon a dissolving or dispersing rate of the detergent, which rate further depends upon a speed at which the wash liquid is agitated, that is, the movability of the clothes in the wash tub. Particularly, in the above-described latter arrangement wherein the time period of the wash step determined mainly in accordance with the detergent concentration, the detergent concentration is not uniform in the wash tub or it takes different values depending upon a volume of the clothes or the cloth quality. Consequently, the time period of the wash step determined in accordance with the detected turbidity of the wash liquid varies widely, which reduces the reliability of the determined time period of the wash step.
Furthermore, the cleaning action of the detergent or the wash liquid is maintained yet while the turbidity is actually changing even upon lapse of the set wash step time period. In this case, it would be better to extend the period of the washing operation. However, this has been ignored in the prior art.